(1) Field of the Invention
The present invention relates to a waveguide type optical non-reciprocal circuit that is simple to implement, is low price, has excellent stability, and that does not use optically non-reciprocal material such as magneto-optical material.
(2) Description of the Related Art
The development of ultra long distance optical fiber communication technology trying to cover over ten thousand kilometers using optical fiber amplifiers, or the investigation of a suitable form of optical fiber technology for transmitting integrated system digital service of a wide band-width, including HDTV (High Definition Television), to the terminal of subscribers, is more than ever being strongly investigated as part of the development of high level optical communication technology. In order to realize this kind of system, one of the indispensable optical components is an optical isolator or circulator, inserter, etc., that is, optical non-reciprocal circuits having optical transmission properties with strong directionality. That is, an optical isolator is a component that prevents deterioration of the signal-to-noise (S/N) ratio which occurs due to the light returning from fiber connection points re-entering the semiconductor laser, which is the optical transmission source. The optical circulator or optical inserter is the optical non-reciprocal circuit used in order to input excitation light into the optical fiber amplifier independently of the optical signal at the time when the optical signal is led into the optical fiber amplifier to be amplified.
A widely practically used example of a known form of the composition of an optical isolator is the non-reciprocal polarizing circuit where a magneto-optical crystal exhibiting the Faraday effect is used as 45.degree. polarization rotator. This optical isolator consists of a minimum of four parts, including the magnet, and it necessitates many processing steps in material fabrication and construction, and has poor reliability and stability. Also, more than anything else, in order to insert such an optical isolator en-route into an optical fiber whose optical polarization state is not necessarily constant, the structure must be such that its device characteristics do not vary with polarization, and for this reason, it requires even more parts and complicated optical circuit structures.
A known example of the optical inserter is one in which, in the case where the wavelength of the inserted light and another passive inserted light differ, a wavelength filter constituted by an interference film is introduced, the passive inserted light is transmitted straight through, the inserted light is reflected through 90.degree. by the wavelength filter, and the waves are coupled. Furthermore, in the case where the wavelength of the inserted light and that of the passive inserted light are the same, a structure employing coupling of optical waves by polarization is known. In these examples, the passive inserted light transmitted through the optical fiber is orthogonally polarized and coupled with the inserted light by a polarizing prism. Thus, an optical circulator or an optical inserter has not yet been realized, which is polarization independent, that is, both the inserted light and the passively inserted light move with no relationship to the condition of polarization.
In the aforementioned cases, the optical circuit is constituted by a beam and not by using an optical waveguide, but the situation is the same even in the case where an optical waveguide is used.